1. Field of the invention
The invention relates to a method for compensating phase noise, generated by lasers, in a coherent optical communication system, which comprises an optical transmitter with a transmitting laser for generating an HF information signal which is formed by a carrier wave modulated by information, as well as for generating an HF pilot signal which is formed by an unmodulated carrier wave, which two HF signals are transmitted via a transmission medium, an optical receiver for receiving said HF signals via said transmission medium, which receiver is provided with a heterodyne circuit comprising a local laser for generating a local HF signal, a mixer circuit for mixing this local HF signal with the HF signals received, and at least one MF filter circuit for letting pass only an MF information signal corresponding to the HF information signal and an MF pilot signal corresponding to the HF pilot signal. The abbreviations "HF ", "MF" and "LF" used above and likewise the corresponding abbreviations "hf", "mf" and "lf" used in the drawings, respectively signify "high frequency", "medium frequency" and "low frequency". These terms are common for describing heterodyne detection, although "intermediate frequency" is perhaps more widely used than "medium frequency". The HF frequency range accordingly means the frequency range a signal before heterodyne detection and the LF frequency range signifies the frequency range of a detected signal after heterodyne detection.
2. State of the art
The performances of coherent optical communication systems can be seriously affected by the phase noise of the lasers. In consequence of this the achievable signal/noise ratio (or "bit error rate") will generally be limited and the sensitivity of the receiver will degrade. The linewidth (spectral bandwidth) of DFB-lasers (DFB: Distributed Feedback) is of the order of 10 MHz and for MEC-DFB-lasers (MEC: Monolitical External Cavity) of the order of 1 MHz. Linewidths in the kHz-area can be realized by means of long external cavities; suchlike large lasers are, however, not suited for being used on a large scale in coherent communication networks.
The fact that in high-quality coherent systems DFB-lasers can only be used for transmission speeds of more than some Gbits/s is caused by the phase noise in such lasers. Moreover, the synchronous modulation will be seriously hindered by the practical implementation of PLLs with the required natural frequencies (some 100 MHz).
From the above it will be clear that techniques for suppressing phase noise in coherent optical communication systems are of great importance. These techniques can be subdivided into:
spectral purification [I], in which case reduction of phase noise is effected directly at the laser sources;
compensation [II], in which case the phase noise at the receiving side is suppressed, after heterodyne detection, by means of signal processing.
[I]Spectral purification can be achieved by optical feedback [a], electrical feedback [b] and optical filtering [c].
[a]Optical feedback is used in extended cavity structures;
[b]Electrical feedback stabilizes the laser using optical frequency discrimination (AFC) [ref. 1, 2]. Although a narrowing of the linewidth can be achieved, the phase noise reduction is restricted to frequencies within the bandwidth of the feedback circuit [ref. 3]. Consequently, the performances of such a coherent system will remain limited [ref. 4];
[c]Optical filtering cuts off the phase noise in the spectral sidebands of the optical carrier wave by means of a band-pass filter (e.g. a glass-fibre filter according to Fabry-Perot).
[II]Compensation techniques make use of an unmodulated pilot-carrier wave (pilot signal) which is sent along with the signal modulated by information (information signal). The pilot signal can be derived from the transmitting laser preceding the modulation by the information to be transmitted. By means of a frequency shift with regard to the carrier wave of the information signal it will be possible to separate at the pilot signal from the information signal at the receiving side. Another possibility is orthogonal polarization of the information signal and of the pilot signal with respect to each other [7]. After heterodyne detection -- by means of the local laser -- phase-noise compensation will be effected by means of signal processing. For this purpose it is known to make use of non-linear heterodyne detection. In this case the two MF signals -- coming from the HF information signal and from the HF pilot signal -- are multiplicatively mixed in a mixer. After unwanted mixture products have been filtered away by a filter, the result will be a phase-noise compensated MF signal; the elimination of the phase noise is the result of the fact that the filter lets pass only the MF mixer signal in which the respective noise components -- mathematically represented -- cancel out each other. After this the 13 low phase-noise - MF signal will be demodulated into an LF signal [ref. 5, 6] in the known way. A drawback of the known compensation method is the relatively large bandwidth required because of the fact that the frequency distances between the HF information signal, the HF pilot signal and the signal of the local (heterodyne) laser have to be so great that the unwanted mixture products can be filtered out. This relatively large bandwidth has especially repercussions on the properties of the HF amplifier of the receiver.